1. Field of the Invention
This invention relates to antenna apparatus and particularly to such apparatus using gyrotropic materials such as ferrites to provide improved directivity.
2. Prior Art
A gyrotropic medium is one whose properties are isotropic in the absence of a magnetic field, but which exhibits anisotropy, i.e. quantitative differences in measurements of the same properties along different crystal axes, when such a field is applied. This characteristic arises in ferrite materials as a consequence of the effect of the field on the electron spin component of the magnetic moment of the atomic lattice, and is the basis of the non-reciprocal properties which are extensively used in ferrite circulators and isolators and many other waveguide and microstrip circuit elements at microwave frequencies.
The continual decrease in size, and increase in power and sensitivity, of solid state transmitters and receivers in the microwave frequency now makes the relatively large size of the antenna structure required in many microwave systems an inconvenient anomaly. While devices using the well-established techniques of phased arrays and aperture synthesis may be utilised to reduce structure dimensions, a lower limit remains which is essentially determined by the wave length of the radiation and the dimensions of the radiating aperture.
U.S. Pat. No. 4,573,055 to Wright describes some proposed arrangements for harnessing the properties of gyrotropic materials to make useful antennas with high angular discrimination, but does not specify how the performance of existing antenna designs may be improved. In U.S. Pat. No. 4,573,055 segments of gyrotropic material are arranged in a shell around a conical magnetic pole piece which introduces a magnetic field which is generally perpendicular to the plane of the shell. Incident radiation is propagated in the segments of gyrotropic material to be extracted at the base of the cone.
U.S. Pat. No. 2,973,516 to Medved discloses a scanning antenna having a length of waveguide with an end closed by a wafer of ferrite material. An electromagnet assembly provides adjustable orthogonal magnetic fields in the plane of the ferrite wafer. Received signals are focussed on to the ferrite wafer by a Luneberg lens. Adjustment of the magnetic fields is said to deflect the antenna beam.